Infrared gas sensors are based on the principle of the selective absorption of infrared radiation by gases. The action principle and the operating manner of infrared gas sensors may be assumed as known.
Such gas sensors generally comprise one or more radiation sources, e.g. thermal radiators such as incandescent lamps, one or more absorption sections, wavelength-selecting elements (selective radiation filters such as e.g. interference filters) and one or more radiation detectors, which convert the optical signal into an electrical measurement signal. A large number of such detectors is known from prior art. The most frequently used types of detector are inter alia pyroelectric, semiconductor (e.g. on a PbSe base) thermophile and pneumatic detectors.
A further type of embodiment is based on the alteration in pressure as a result of the heating of the gas molecules in the measurement area, which is detected by means of a microphone. The heating is caused by the absorption of the radiation energy of the radiation source(s) by the measurement gas molecules to be detected. An embodiment corresponding to prior art is disclosed in the patent DE 195 25 703 A1.
Thus, U.S. Pat. No. 5,608,219 teaches a device for detecting at least one gas with an absorption band in the infrared range, comprising a cell which contains the gas sample to be tested, an infrared radiation source, a power supply for the radiation source, an infrared radiation sensor and a measured-value evaluation unit which is secured to the sensor.
EP 512 535 teaches a portable carbon dioxide monitor in which the carbon dioxide is determined by non-dispersive infrared measurements.
EP 503 511 teach a device for the analytical determination of carbon dioxide and water through infrared analysis techniques. The gas analyzer here contains a radiation source, a sample cell and a reference cell, a detector and a gas supply.
U.S. Pat. No. 5,734,165 describes an infrared photometer which is integrated in a compact housing in which an infrared radiation source is provided with a pulse frequency of between 0.01 and 10 Hz.
U.S. Pat. No. 4,914,720 teaches a gas analyzer which is based on non-dispersive infrared photometry and with which different gases in gas mixtures can be determined.
In optical gas sensors, the radiation source is generally operated continuously (in combination with a chopper) or intermittently with a pulsed voltage. In both cases, the signal is generally detected by means of a phase-sensitive electronics (lock-in technology) or respectively an RMS converter.
The invention relates to an infrared gas sensor with an energy supply apparatus for operating at least one radiation source, for example a heat radiation source or an infrared luminescence radiation source (e.g. diode, laser diode, infrared laser), with current or voltage pulses, with at least one measurement area arranged in the beam path with at least one detector emitting an electrical measurement signal and with one switching device to control the pulse duration of the current or voltage pulses. The invention relates furthermore to a method of operating such a sensor.
In a gas sensor of this type, known from DE 30 43 332 A1, the pulse duration of the radiation source, a heat radiator, is controlled in dependence on the maximum value of the measurement signal of a radiation detector at the exit of the measuring transducer. It is proposed here that the maximum value be obtained empirically or mathematically from the course of the response curve or of the measurement signal of the measuring transducer, and that the pulse duration be controlled, by interrupting the energy supply to the heat radiator, in such a way that the maximum value lies with certainty within the pulse duration. Furthermore it is proposed that the course of the response curve be monitored by means of a maximum value detector which interrupts the energy supply or respectively the current pulse when the maximum value of the response curve is reached. With this gas sensor, a pulse/pause ratio of the energy supply of  less than 1 and preferably between 0.1 and 0.5 can be achieved.
The object underlying the invention is to create a gas sensor of this type which has a reduced energy consumption with high measuring accuracy.
This object is achieved in relation to the sensor by the features of patent claim 1 and in relation to the method by the features of claim 9. The subordinate claims list advantageous developments.
According to the invention, therefore, the switching device is so controlled that the pulse duration is smaller than the time interval until the measurement signal has reached the maximum at xcfx84MAX. This can come about by the maximum rise of the detector signal       (                  ⅆ                              U            0                    ⁡                      (            t            )                                      ⅆ        t              )    MAX
being used as the measured value. The maximum rise of the signal is achieved substantially quicker than its maximum value: xcfx84xe2x80x2 less than  less than xcfx84MAX. Therefore the turn-on time of the radiation source xcfx840 in this measurement process can be substantially shorter xcfx84xe2x80x2xe2x89xa6xcfx840 less than  less than xcfx84MAX than in all the previous ones. The pulse/pause ratio which can be thus realised is below 0.01.
The interval between individual measurements can be selected according to sensor design and application profile between a few seconds and several minutes, whereby it is possible for an operating period without a change of batteries (e.g. standard AA batteries) of more than half a year to be achieved.
This method according to the invention, like the methods according to the invention presented below, can be carried out both with a radiation detector and with an acoustic detector, such as e.g. a microphone, as the detector element. Simultaneously with a plurality of detectors, e.g. a radiation detector and a microphone, a plurality of signals can be produced, the measurement being carried out according to the invention for at least one of the two signals or also for a plurality of the signals or respectively for all the signals. When a plurality of signals is produced, the accuracy of the measurement can be further improved.
Suitable as radiation sources for infrared rays are for example incandescent lamps, thermal thin-film radiators, light emitting diodes and the like which can also be used as pulsed infrared radiation sources.
Another method of shortening the turn-on time of the radiation source, is using the measurement of the detector signal at a specific time xcfx84 after turning on the radiation source as the sensor signal, this time xcfx84 being smaller than the time of reaching the maximum value of the detector signal xcfx84MAX: xcfx84 less than xcfx84MAX. Preferably xcfx84 lies between the time of reaching the maximum value of the rise of the detector signal xcfx84xe2x80x2 and xcfx84MAX: xcfx84xe2x80x2xe2x89xa6xcfx84 less than xcfx84MAX. As the measured value at the time xcfx84, both the instantaneous value of the detector signal UD(xcfx84) and the instantaneous value of its first derivative can be used       (                  ⅆ                              U            D                    ⁡                      (            t            )                                      ⅆ        t              )        τ    =          τ      xe2x80x2      
Furthermore it is also possible to use as the measured value the period of time after the turn-on of the radiator in which a specific level of the detector signal Uxe2x80x3 or a specific spacing from the detector signal when the radiator is turned off (offset) is reached.
The period of time until a fixed rise of the detector signal is reached can also be used as the measurement signal.
Very advantageous is the operating manner of the gas sensor when the maximum rise of the detector signal       (                  ⅆ                              U            D                    ⁡                      (            t            )                                      ⅆ        t              )    MAX
or the time at which this maximum value is reached xcfx84xe2x80x2 is used as the measurement signal. Here the maximum rise can be measured with the aid of a maximum value detector without giving the time of measurement.
The time xcfx84xe2x80x2 can be determined e.g. with the aid of a zero crossing detector from the second derivative of the detector signal.
Measuring the integral of the detector signal up to a specific point of time xcfx84, e.g. up to the maximum of the rise of the detector signal xcfx84xe2x80x2, is a further possible measuring method.
Preferably that is an integral over a period of time starting from the turn-on point of the radiation source (t=0) up to a specific time xcfx84 which is smaller than the pulse duration xcfx84 less than xcfx840:       ∫    0    τ    ⁢                    U        D            ⁡              (        t        )              ⁢                  ⅆ        t            .      
Naturally also a plurality of the above-described methods and measured values (e.g.       (                  ⅆ                              U            D                    ⁡                      (            t            )                                      ⅆ        t              )    MAX
and xcfx84xe2x80x2) can be combined with one another to increase the sensitivity or the reliability.
The invention also relates to a method of operating an infrared gas sensor such as has been explained in detail above.
The method according to the invention is carried out in such a way that the current or voltage pulse is so turned off that the pulse duration is smaller than that required to reach the maximum (xcfx84MAX) of the measurement signal of the radiation detector(s) or of the acoustic detector. Preferred embodiments of the method consist in the current or voltage pulse being turned off after a time t=xcfx84, xcfx84 lying between t=0 of the current or voltage pulse and xcfx84MAX. It is particularly preferred if the current or voltage pulse is turned off at the maximum value of the first derivative of the measurement signal within the pulse duration. But all the embodiments previously explained in connection with the sensor can be used in the method according to the invention.
The method according to the invention has additional advantages. It is thus also possible with the method according to the invention to determine the gas concentration in a different manner. Thus the gas concentration can be determined from the value of the first derivative of the measurement signal at a specific time xcfx84mexcex2, which is smaller than the pulse duration or at a value of the N-th derivative (N greater than 1) of the measurement signal at a specific time xcfx84mexcex2, which is smaller than the pulse duration. The gas concentration can furthermore be determined from the value of the integral of the measurement signal over a period of time starting from the turn-on time t=0 up to a specific time xcfx84mexcex2 which is smaller than the pulse duration. The first derivative can also be used for determining the gas concentration. Further preferred embodiments of the method for determining the gas concentration are quoted in claims 16 to 19.
The method according to the invention can also be used when the time constant of the detector is greater than the time constant of the radiation source used. It includes furthermore measuring methods and measuring arrangements suitable for same, in which the measurement signal dependent on the gas concentration is determined before the maximum of the detector signal is reached. In particular the pulse length of the voltage applied to the radiation source to operate same can be kept so short that the maximum of the detector signal is not reached. For frequently the kinetic course of the detector output signal, e.g. in the case of pyroelectric detectors, is determined more strongly by the properties, such as for example the time constant, of the detector itself than by the properties of the radiation source.